As new generations of handsets, and other wireless communication devices become embedded with more applications and complexity, there is a need for ever more integration. The trend in mobile radio communications is towards complex multi-radio systems comprised of several parallel transceivers. This implies a leap in complexity of the radio frequency (RF) front-end (FE) design. The RF circuits of wireless communication devices, and the transmitter parts in particular, are difficult to integrate.
Known transmitter architectures create undesired harmonics of transmit signals, due to the nonlinearity of each transmitter stage, e.g. an analog quadrature baseband circuit, an up-conversion mixer stage, the power amplifier (PA) stage, etc. This results in harmonic RF spurs being generated at the transmit output, which may not comply with out-of-band transmission specifications of wireless communication standards and thus impact the communication transmission and reception of other wireless communication units. Alternatively, or additionally, they may cause self-interference in other transceiver paths implemented in the same communication unit.
In particular, spurs may be generated around the frequency of the wanted/desired transmit signal, ω0+ωbb, that are within the transmit band. Such undesired spurs and harmonics include the baseband image frequency at the RF, ω0−ωbb, the local oscillator leakage, ω0 LO leakage, and multiple counter inter-modulation products (referred to as CIM spurs), such as third, fifth, seventh, . . . harmonics of the baseband signal located around the wanted/desired transmit signal, ω0−3ωbb, ω0+5ωbb. In particular, CIM spurs around the wanted signal severely degrade performance such as adjacent channel leakage rejection (ACLR) and spurious emissions. Counter 3rd order and 5th order intermodulation (CIM3/CIM5) components are known to be the most critical ones to cancel or remove, with the higher order CIM products being less significant.
Harmonic mixing at different TX stages generates/regenerates CIM products. As the harmonics are regenerated at each TX stage, it is necessary to suppress CIM products. Notably, due to the problematic effect that each active stage regenerates CIM products, even if they have been substantially cancelled or removed earlier in the transmit chain, all the stages of the transmitter need to be considered when attempting to remove the generated/regenerated CIM products.
Four known solutions of: Weldon, He, Vora and Ingels attempt to reduce harmonic spurs, each of which are based on essentially the same idea of sinewave approximation. By adding multiple signals with different phases and amplitudes, a first order sinewave approximation can be achieved. An amplitude scaling of √2 allows the use of phase shifts that are easy to generate and hence each of the known art that is identified below use √2 amplitude scaling in the signal path. One disadvantage common to these four solutions is that RF signals with different phases are combined directly, inevitably leading to power loss and hence reduction of transmitter efficiency.
FIG. 1 illustrates a known quadrature transmitter architecture 100. The transmitter architecture 100 comprises a quadrature (I/Q) baseband input signal 110. The I/Q baseband input signal 110 is input to quadrature up-mixer 130 via a respective low pass filter 120, which up-converts the I/Q baseband signal 110 in response to respective quadrature local oscillator (LO) signals 125, 135, there being a 90 degree phase shift between the respective quadrature LO signals. The up-converted quadrature signals are amplified in RF amplifiers 140 and both paths are summed at combiner 150. The combined signal is then amplified in power amplifier 170.
FIG. 2 provides a graphical illustration 200 of a wanted signal 210 and a multitude of spurs and harmonics that are created in known quadrature transmitters, that require careful attention when designing a transmitter architecture. The illustrated spurs and harmonics include LO leakage 226 (ω0), image spur 224, CIM3 spur 220 (ω0−3ωbb) and CIM5 spur 232 (ω0+5ωbb), as well as other 2nd harmonic spurs 222 and 228 and 3rd harmonic spur 230.
A transmitter architecture by M. Ingels et al., as described in “A multiband 40 nm CMOS LTE SAW-less modulator with −60 dBc C-IM3”, published in the ISSCC digest of technical papers, p 338-339, February 2013, targets rejection/cancellation of only the C-IM3 product. For example, in Ingels, the CIM3 products from the three paths have three phases, 0, −90°, 135°. With a scaling factor of √2, the CIM3 tone is cancelled when the three signals are summed. Thus, Ingels (as well as other known techniques) describes an architecture that is capable of careful manipulation of quadrature signals to effect cancellation of one undesired transmit harmonic or spur, unfortunately leaving other strong harmonics and spurs that fail to meet a specific performance or may re-create undesired spurious emissions.
Therefore, known techniques for reducing or cancelling harmonic spurs, including CIM products, are less than ideal in that other harmonic spurs, including CIM products, are generated at sufficient levels to cause or potentially cause spurious emission issues.